Circuits and method for controlling transient fault conditions in a low dropout voltage regulator

ABSTRACT

An overshoot reduction circuit within a low dropout voltage regulator eliminates an overshoot at an output terminal resulting from a transient fault condition occurring at an input or output terminal. The overshoot reduction circuit monitors to sense if there is a transient fault condition occurring at the input or output terminal and provides a Miller capacitance at the output terminal of a differential amplifier of the low dropout voltage regulator to prevent the output of the differential amplifier from being discharged to ground during the transient. A control loop circuit balances current within an active load of the differential amplifier to clamp the output of the differential amplifier to its normal operating point. When the transient fault condition ends, the output voltage of the differential amplifier is set such that a pass transistor of the low dropout regulator responds quickly to resume the regulation to reduce or eliminate the overshoot.

TECHNICAL FIELD

This disclosure relates generally to voltage regulator circuits. Moreparticularly, this disclosure relates to low dropout voltage regulatorsand even more particularly to low dropout voltage regulators that havecircuitry that maintains regulation of an output voltage of the lowdropout voltage regulator during transient fault conditions.

BACKGROUND

As is known in the art, a voltage regulator is a constant voltage sourcethat adjusts its internal resistance to any occurring changes of loadresistance to provide a constant voltage at the regulator output. FIG. 1is a schematic diagram of a low dropout voltage regulator. The loadresistance of the voltage regulator as shown is formed by the parallelcombination of the equivalent series resistance R_(ESR) of the loadcapacitor C_(L) and the load resistor R_(L).

In order to regulate the output voltage resulting from any changes isthe load resistor R_(L), the internal resistance of the voltageregulator must be adjusted to maintain the output voltage V_(OUT) at thedesired level. To accomplish this, the output voltage is sensed by thevoltage divider formed by the series resistors R₁ and R₂. As is known,the feedback voltage V_(FB) is the product of the output voltage V_(OUT)and the ratio of the resistor R₂ and the sum of the series resistors R₁and R₂. An error amplifier receives the feedback voltage V_(FB) andcompares it with a reference voltage V_(REF). The output voltage of theerror amplifier A_(ERR) is an indication of the error between thefeedback voltage V_(FB) and the reference voltage V_(REF) that isapplied to gate of the PMOS pass transistor P_(PASS). Thedrain-to-source voltage (Vds) and the drain-to-source current (Ids)determine the equivalent internal resistance of the low dropout voltageregulator. As is known, the drain-to-source voltage (Vds) and thedrain-to-source current (Ids) are determined by the transconductance ofthe PMOS pass transistor P_(PASS) and the gate-to-source voltage (Vgs)of the PMOS pass transistor P_(PASS).

The dropout voltage of the low dropout regulator is normally defined thepoint at which the drain-to-source voltage (Vds) of the PMOS passtransistor P_(PASS) is not changed when the gate-to-source voltage (Vgs)changes and the PMOS pass transistor P_(PASS) is in saturation.

The size of the PMOS pass transistor P_(PASS) is normally very large toprovide the necessary current to the load resistance R_(L). Further theload capacitance C_(L) and the miller capacitance of the PMOS passtransistor P_(PASS) create a zero the right hand plane that may causeinstability in the error amplifier A_(ERR) and cause oscillation in theoutput voltage. To alleviate the instabilities, the compensationcapacitor C_(COMP) is placed between the gate and the drain of the PMOSpass transistor P_(PASS) to shift the zero sufficiently high infrequency to not cause the instabilities.

In the existing low dropout regulators, a line transient at the inputvoltage terminal V_(IN) may cause the low dropout regulator to go deepinto dropout. When the transient is removed from the input terminalV_(IN), the output of the error amplifier A_(ERR) is at a voltage levelsuch that the PMOS pass transistor P_(PASS) is deep into triode regionof operation. This causes a very large overshoot voltage level to occurat the output terminal V_(OUT) and across the load capacitance C_(L) andthe load resistance R_(L). The very large overshoot voltage level cancause reliability issues for a load device forming the load capacitanceC_(L) and the load resistance R_(L) being powered by low dropoutregulator.

SUMMARY

An object of this disclosure is to provide a circuit that maintainsregulation of an output voltage at the output terminal of a low dropoutvoltage regulator at the removal of transient fault conditions at theinput terminal or output terminal of the low dropout voltage regulator.

Another object of this disclosure is to provide a control loop circuitfor balancing an active load current in an error amplifier of a lowdropout voltage regulator resulting from transient fault conditionsoccurring at an input terminal or an output terminal of the low dropoutvoltage regulator.

To accomplish at least one object of this disclosure, a control loopcircuit balances an active load current in an error amplifier of a lowdropout voltage regulator resulting from a transient fault conditionsoccurring at an input terminal or an output terminal of the low dropoutvoltage regulator to minimize or eliminate an overshoot at an outputterminal of a low dropout voltage regulator resulting from the transientfault conditions occurring at an input terminal or an output terminal ofthe low dropout voltage regulator. The control loop circuit has atransient fault sense circuit to determine that the low dropout voltageregulator is responding to input or output transient faults. A localcontrol loop is connected to the output terminal of the sense circuit toreceive a transient fault indicator signal communicating that the lowdropout voltage regulator is responding to input or output transientfaults. If the transient fault indicator signal shows that the lowdropout voltage regulator is responding to input or output transientfaults, the local control loop clamps an output voltage level of adifferential amplifier of an error amplifier close to the operatingpoint of the error amplifier. The local control loop clamps the outputvoltage level by balancing the currents in an active load of thedifferential amplifier, while a pass transistor is in the deep trioderegion of operation. When the transient voltage present at the inputterminal or a transient load current present at the output terminal ofthe low dropout voltage regulator is ending, the voltage level presentat the input terminal is returned to its normal operating voltagewithout any overshoot.

The transient fault sense circuit has an input terminal connected to anoutput driver of the low dropout voltage regulator to indicate that thelow dropout voltage regulator is responding to input or output transientfaults. The transient fault sense circuit has a first transistor of afirst conductivity type (PMOS) having a source terminal connected to asense point within the output driver of the low dropout voltageregulator. A gate of the first transistor of the first conductivity typeis connected to a reference voltage source such that when the sensepoint increases sufficiently large the first transistor of the firstconductivity type is conducting. The conducting of the first transistorof the first conductivity type indicates that the voltage level at theoutput of low dropout voltage regulator is below the required regulatedvoltage and the low dropout voltage regulator is responding to input oroutput transient faults. The drain of the first transistor of the firstconductivity type is connected form the output terminal of the transientfault sense circuit and is connected to the local control loop toprovide the transient fault indicator signal to the local control loop.The output terminal of the local control loop is connected to the outputterminal of the differential amplifier. The drain of the firsttransistor of the first conductivity type is connected to a gate anddrain of a first transistor of a second conductivity type (NMOS). Thesource of the first transistor of the second conductivity type isconnected to the ground reference voltage source. The first transistorof the second conductivity type acts as a load transistor of the firsttransistor of the first conductivity type. The drain of the firsttransistor of the first conductivity type is further connected to a gateand drain of a second transistor of the second conductivity type and toa first terminal of a constant current sink.

The second terminal of the constant current sink is connected to theground reference voltage source. The source of the second transistor ofthe second conductivity type is connected to the ground referencevoltage source. The constant current sink insures that the gates anddrains of the first and the second transistor of the second conductivitytype are forced to the voltage level of the ground reference voltagesource when the first transistor of the first conductivity type is notconducting.

The drain of the second transistor of the second conductivity type isconnected to a first terminal of a constant current source. The secondterminal of the constant current source is connected to input terminalof the low dropout voltage regulator. The common connection between theconstant current source and the drain of the second transistor of thesecond conductivity type is connected to the output terminal of thetransient fault sense circuit and thus to the input of the local controlloop. The input of the local control loop is connected to a gate of asecond transistor of the first conductivity type within the localcontrol loop. The drain of second transistor of the first conductivitytype is connected to output terminal of the differential amplifier. Thesource of the second transistor of the first conductivity type isconnected to the drain of a third transistor of first conductivity type.The source of the third transistor of the first conductivity type isconnected to a biasing current source of the differential amplifier ofthe error amplifier of the low dropout voltage regulator. The gate ofthe third transistor of the first conductivity type is connected toreceive a feedback voltage that is scaled from the voltage level of theoutput terminal of the low dropout voltage regulator.

When a transient fault condition occurs at an input terminal or anoutput terminal of the low dropout voltage regulator, the output of thedifferential amplifier decreases and the voltage at the source of thefirst transistor of the conductivity type increases. The firsttransistor of the first conductivity type begins to conduct and thesecond transistor of the second conductivity type begins to conductcausing the voltage at the gate of the second transistor of the firstconductivity type to decrease dramatically as the second transistor ofthe second conductivity type begins to saturate. The second transistorof the first conductivity type is conducting. The voltage differencebetween the feedback voltage level and the voltage level of the inputterminal of the low dropout voltage regulator causes the thirdtransistor of the first conductivity type to conduct also. This causesthe output voltage level of the differential amplifier to be clampedclose to is operating level rather than ground by balancing the currentin the active load of the differential amplifier. This allows the lowdropout voltage regulator to assume regulation without any overshootoutput voltage when the transient at the input terminal of the lowdropout voltage regulator is removed.

In various embodiments, at least one object of this disclosure isaccomplished by a low dropout voltage regulator that includes a controlloop circuit that balances an active load current in an error amplifierof a low dropout voltage regulator resulting from a transient faultcondition occurs at an input terminal or an output terminal of the lowdropout voltage regulator to minimize or eliminate an overshoot at anoutput terminal of a low dropout voltage regulator resulting from thetransient fault conditions occurring at an input terminal or an outputterminal of the low dropout voltage regulator. The control loop circuithas a transient fault sense circuit to determine that a transient faultcondition. A local control loop is connected to the output terminalsense circuit to receive a transient fault indicator signalcommunicating the low dropout voltage regulator has lost regulation andthe voltage at the output of the low dropout voltage regulator is lowerthan the required regulated voltage. If the transient fault indicatorsignal shows that the low dropout voltage regulator is in responding toinput or output transient faults, the local control loop balances thecurrent in the active loads of the differential amplifier and clamps anoutput of a differential amplifier of the error amplifier close to theoperating point of the differential amplifier. When the transientvoltage present at the input terminal or the transient current presentat the output terminal of the low dropout voltage regulator is ending,the voltage level present at the input terminal is returned to itsnormal operating voltage with no overshoot.

In various embodiments, at least one object of this disclosure isaccomplished by a low dropout voltage regulator that includes a localcontrol loop connected to provide a balancing current to an active loadof a differential amplifier within the low dropout voltage regulatorwhen a transient fault condition occurs at an input terminal or anoutput terminal the low dropout voltage regulator. The balancing currentin the active load of the differential amplifier clamps an outputvoltage level of the differential amplifier near an operational voltagelevel when a pass transistor of the low dropout voltage regulator isdeep triode region of operation or the low dropout voltage regulator isin current limit condition or any other fault condition that forces theoutput voltage of the low dropout voltage regulator to be lower than therequired regulated voltage. Maintaining the output voltage level nearthe operational voltage level insures fast response in controlling thegate of the pass transistor to minimize overshoot when the inputterminal of the low dropout voltage regulator is ending and the voltagelevel present at the input terminal is returned to its normal operatingvoltage.

The local control loop has a clamping transistor of a first conductivitytype, a diode connected transistor of the first conductivity type, and abiasing transistor of the second conductivity type. The clampingtransistor has a source connected to a driver circuit of the low dropoutvoltage regulator and a drain connected to an output of the differentialamplifier of the low dropout voltage regulator to provide the balancingcurrent to the active load of the differential amplifier. The balancingcurrent forces an output voltage at an output terminal of thedifferential amplifier to be at approximately \ the operating voltagelevel of the differential amplifier. The diode connected transistor ofthe first conductivity type has a source connected to a pass supplyvoltage source, a gate and drain connected to a gate of the clampingtransistor. The biasing transistor of a second conductivity type has adrain connected to a drain of the diode connected transistor and thegate of the clamping transistor. The gate of the biasing transistor isconnected to receive a biasing voltage level and a source of the biasingtransistor is connected to ground reference voltage source.

A voltage developed by the diode connected transistor and the biasingtransistor at the gate of the clamping transistor is less than thevoltage level at the source of the clamping transistor. When the passsupply voltage source has a transient voltage, the clamping transistoris turned on to provide the balancing current to the active load of thedifferential amplifier. The balancing current sets the output voltage atan output terminal of the differential amplifier is at approximately atthe operating voltage level of the differential amplifier to quicklycontrol a gate of a pass transistor of the low dropout regulator tominimize an overshoot of an output voltage of the low dropout voltageregulator.

In various embodiments, at least one object of this disclosure isaccomplished by a method performed by an overshoot reduction circuitwithin a low dropout voltage regulator that begins at the activation ofthe low dropout voltage regulator. An input voltage at the inputterminal of the low dropout voltage regulator is monitored to sense ifthere is a transient fault condition occurring at an input terminal oran output terminal. When the transient fault condition is present at theinput terminal or the output terminal of the low dropout voltageregulator, the overshoot reduction circuit balances a current within anactive load of a differential amplifier of an error amplifier of the lowdropout voltage regulator to clamp the output of the differentialamplifier to its normal operating point. When the transient faultcondition ends, the output voltage of the error amplifier is set suchthat the pass transistor responds quickly to resume the regulation andthe overshoot is reduced or eliminated and the sensing of the inputvoltage level is sensed for the presence of the transient. The currentbalancing ensures that when the low dropout voltage regulator isresponding to input or output transient faults, the plate of a Millercapacitance at the output terminal of the differential amplifier is notdischarged to the ground reference voltage level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a low dropout voltage regulator.

FIG. 2 is a schematic diagram of a low dropout voltage regulator of FIG.1 showing a detailed schematic of the error amplifier.

FIGS. 3a-3e are plots of voltage levels at various connection pointswithin the low dropout voltage regulator of FIG. 2 during a voltagetransient fault condition of the input voltage.

FIGS. 4a-4f are plots of voltage and current levels at variousconnection points within the low dropout voltage regulator of FIG. 2during a 50 ma current transient fault condition of an output loadcurrent of an accessory connected to the low dropout voltage regulatorof FIG. 2.

FIGS. 5a-5f are plots of voltage and current levels at variousconnection points within the low dropout voltage regulator of FIG. 2during a 200 ma current transient fault condition of an output loadcurrent of an accessory connected to the low dropout voltage regulatorof FIG. 2.

FIG. 6 is a schematic diagram of a low dropout voltage regulator of animplementation embodying the principals of the present disclosure.

FIGS. 7a-7f are plots of voltage levels at various connection pointswithin the low dropout voltage regulator of FIG.6 during a voltagetransient of the input voltage.

FIG. 8 is a schematic diagram of a low dropout voltage regulator ofanother implementation embodying the principals of the presentdisclosure.

FIGS. 9a-9e are plots of voltage levels at various connection pointswithin the low dropout voltage regulator of FIG.8 during a voltagetransient of the input voltage.

FIGS. 10a-10f are plots of voltage and current levels at variousconnection points within the low dropout voltage regulator of FIGS. 6and 8 during a 200 ma current transient fault condition of an outputload current of an accessory connected to the low dropout voltageregulator of FIGS. 6 and 8.

FIG. 11 is a flowchart for a method for maintaining regulation of anoutput voltage of the low dropout voltage regulator during transientfault conditions at the input or output terminals of the low dropoutvoltage regulator embodying the principals of this disclosure.

DETAILED DESCRIPTION

FIG. 2 is a schematic diagram of the low dropout voltage regulator ofFIG. 1 showing a detailed schematic of the error amplifier A_(ERR). Theerror amplifier A_(ERR) has a differential amplifier formed of thedifferential pair of PMOS transistors P4 and P5. The PMOS transistor P3forms a biasing current source for the differential amplifier and theNMOS transistors N5 and N6 form the load devices for the differentialpair of PMOS transistors P4 and P5. The PMOS transistors P1 and P2, abiasing current source I_(BIAS), and the NMOS transistor N1 form avoltage biasing generator that generates the P-type biasing voltageV_(PBIAS) and the N-type biasing voltage V_(NBIAS). The P-type biasingvoltage V_(PBIAS) is applied to the gate of the PMOS transistor P3 tobias the PMOS transistor P3 to for generating the biasing current forthe differential pair of transistors P4 and P5.

The drains of the PMOS transistor P5 and the NMOS transistor N6 areconnected to form the output terminal of the differential pair of theerror amplifier A_(ERR). The feedback voltage V_(FB) that is developedat the common connection A of the series resistors R₁ and R₂ is appliedto the gate of the PMOS transistor P4. A reference voltage V_(REF) isapplied to the gate of the PMOS transistor P5. The difference in thefeedback voltage V_(FB) and the reference voltage V_(REF) is developedat the output terminal B of the differential pair of transistors P4 andP5 of the error amplifier A_(ERR) as the error voltage V_(ERR). Thedrain of the PMOS transistor P4 is connected to the drain and gate ofthe NMOS transistor N5 and the gate of the NMOS transistor N6. Thesources of the NMOS transistors N5 and N6 are connected to the groundreference voltage source.

The error voltage V_(ERR) is applied from the output terminal B of thedifferential pair of transistors P4 and P5 to the gate of the NMOStransistor N3. The NMOS transistors N3 and N4 are the output driver forthe error amplifier A_(ERR). The PMOS transistor P7 forms the loadtransistor for the NMOS transistor N3. The diode connected PMOStransistor P6 and the NMOS transistor N2 form the biasing circuit forthe PMOS load transistor P7. The N-type biasing voltage V_(NBIAS) fromthe biasing voltage generator is applied to the gate of the NMOStransistor N2. The sources of the PMOS transistors P6 and P7 areconnected to the input voltage terminal E of the low dropout voltageregulator to receive the input voltage level VDD.

The NMOS transistor N3 and the PMOS transistor P7 form an amplifierstage for amplifying the error voltage V_(ERR) to be applied to the gateof the NMOS transistor N4. The diode connected PMOS transistor P8 is theload transistor for the NMOS transistor N4. The drain of the NMOStransistor N4 is connected to the gate and drain of the PMOS transistorP8. The source of the PMOS transistor P8 connected to the input voltageterminal E of the low dropout voltage regulator to receive the inputvoltage level VDD. The source of the NMOS transistor N4 is connected tothe ground reference voltage source. The common connection of the drainof the NMOS transistor and the gate and drain of the PMOS transistor P8form the output terminal C of the error amplifier A_(ERR). The PMOStransistor P9 is the pass transistor of the low dropout voltageregulator. The gate of the PMOS transistor P9 is connected to the outputterminal of the error amplifier A_(ERR) and thus to the commonconnection C of the drain of the NMOS transistor N4 and the gate anddrain of the PMOS transistor P8 to apply the gating control voltageV_(GATE) to the PMOS transistor P9. The source of the PMOS transistor P9connected to the input voltage terminal E of the low dropout voltageregulator to receive the input voltage level VDD. The drain of the PMOStransistor P9 is connected to the to the output terminal D of the lowdropout voltage regulator to provide the output voltage level V_(LDO) tothe load capacitor C_(L) and the load resistor R_(L) of FIG. 1.

As in FIG. 1, the drain of the PMOS pass transistor P9 is connected tothe top terminal of the series resistors R₁ and R₂. Also, as in FIG. 1,the compensation capacitor C_(COMP) is connected from the commonconnection D of the series resistors R₁ and R₂ and the PMOS passtransistor P9 and the output terminal B of the error amplifier A_(ERR).As is similar to the description in FIG. 1, the compensation capacitorC_(COMP) is placed between output terminal B of the differentialamplifier and the drain of the PMOS pass transistor Pg and the outputterminal D of the low dropout voltage regulator to shift the zerosufficiently high in frequency to not cause instabilities.

The PMOS transistor P11 is an optional clamping transistor. The sourceof the PMOS transistor P11 is connected to the common connection F ofthe drain of the NMOS transistor N3, the gate of the NMOS transistor N4,and the drain of the PMOS transistor P7. The gate of the PMOS transistorP11 is connected to receive the reference voltage V_(REF). The drain ofthe PMOS transistor P11 is connected to the ground reference voltagesource. When the low dropout regulator is responding to input or outputtransient faults and the PMOS pass transistor P9 is in the deep triodeoperation region, the PMOS transistor P11 clamps the voltage at thecommon connection F, but does not prevent the overshoot of the voltagelevel V_(LDO) at the output terminal D.

FIGS. 3a-3e are plots of voltage levels at various connection pointswithin the low dropout voltage regulator of FIG. 2 during a voltagetransient fault condition of the input voltage level VDD applied to theinput voltage terminal E. If a line transient (100 of FIG. 3a ) occursthat causes the voltage level VDD applied to the input voltage terminalE to have a voltage excursion that approaches or exceeds the regulatedoutput voltage level V_(LDO) (102 of FIG. 3e ) present at the outputterminal D, the internal nodes of the error amplifier A_(ERR) will beskewed. In the present example, the transient of the voltage level VDDis from 3.5V to 2.97V and back to 3.5V. The rise and fall times for theinput voltage level VDD are 4 μs. The voltage level V_(ERR) at theoutput terminal B of the differential amplifier will be clamped (104 ofFIG. 3b ) close to ground. As the voltage level V_(Fst1) at the commonconnection F begins to the rise (106 of FIG. 3c ) as result of the NMOStransistor N3 being turned off, the gate-to-source voltage of the PMOStransistor P11 becomes large enough to turn on the PMOS transistor P11.The voltage level V_(FST1) at the common connection F is clamped to avoltage level of approximately 2.4V or the gate-to-source voltage level(Vgs) of the PMOS transistor P11 greater than the reference voltageV_(REF). With the output voltage level V_(ERR) of the differentialamplifier approaching a voltage level such that the NMOS transistor N3is deep in to the triode region of operation, the majority of thecurrent flowing through the PMOS transistor P7 will be flowing throughthe PMOS transistor P11. The voltage level V_(Fst1) causes the NMOStransistor N4 to turn on and thus the voltage level V_(GATE) (108 ofFIG. 3d ) at the output terminal C of the error amplifier A_(ERR)approaches the voltage level of the ground reference voltage source andthe PMOS pass transistor P9 is turned fully on to be in the deep triodeoperating region (110 of FIG. 3e ).

When the line transient is removed (112 of FIG. 3a ) and the voltagelevel VDD applied to the input voltage terminal E returns from theminimum voltage of the transient (2.97V) to the normal applied voltagelevel (3.5V), the error output voltage level V_(ERR) (114 of FIG. 3b )at the output of the differential amplifier takes a finite time to reachits desired operating voltage. During this time the PMOS pass transistorP9 is turned on into deep triode region of operation (116 of FIG. 3d )and will source current to the output terminal D and the load capacitorC_(L) and the load resistor R_(L). This results in large voltageovershoot (118 of FIG. 3e ) of approximately 250 mV at the outputvoltage level V_(LDO). After a period of time the output voltage levelV_(LDO), will return (120 of FIG. 3e ) to the regulated output voltagelevel V_(LDO) of approximately 3.0V. The large voltage overshoot in theoutput voltage level V_(LDO) may cause physical damage to the circuitsand systems that compose the load capacitor C_(L) and the load resistorR_(L).

FIGS. 4a-4f are plots of voltage and current levels at variousconnection points within the low dropout voltage regulator of FIG. 2during a 50 ma current transient fault condition of an output loadcurrent of an accessory connected to the low dropout voltage regulatorof FIG. 2. The low dropout voltage regulator has a regulated outputvoltage level V_(LDO) of approximately 2.4V in this example (122 of FIG.4f ) with a load current I_(LOAD) of approximately 1 ma (124 of FIG. 4a) developed from an input voltage level VDD applied to the input voltageterminal E of approximately 2.7V (126 of FIG. 4b ). The voltage levelV_(ERR) at the output terminal B of the differential amplifier is atapproximately 378 mV (128 of FIG. 4c ). The voltage level V_(Fst1) atthe common connection F is set to the voltage level is approximately 700mV (130 of FIG. 4d ). This forces the voltage level V_(GATE) at the gateof the PMOS pass transistor P9 to be 1.87V (132 of FIG. 4e ).

The accessory of the low dropout voltage regulator increases its loadcurrent I_(LOAD) from the output terminal D to a value of approximately50mA (134 of FIG. 4a ). The causes the feedback loop to modify thepotential of internal nodes of low dropout voltage regulator such thatthe low dropout voltage regulator can supply the 50 mA load current.After a sharp negative voltage spike, the voltage level V_(ERR) at theoutput terminal B of the differential amplifier drops from 378 mV to 373mV (138 of FIG. 4c ). The voltage level V_(FST1) at the node F increasesto a voltage of 1.02V (138 of FIG. 4c ) and the voltage V_(GATE) at thegate of the PMOS pass transistor P9 reduces from 1.87V to 1.42V. Theregulated output voltage level V_(LDO) present at the output terminal Dspikes negatively to a voltage level of approximately 2.38V for a briefperiod of time and then resumes control at the regulated voltage levelof approximately 2.40V (146 of FIG. 4f ). The input voltage level VDDapplied to the input voltage terminal E decreases by a voltage ofapproximately 15 mV to approximately 2.685V (136 of FIG. 4b ).

When the accessory no longer requires the load current of 50 mA, theload current I_(LOAD) is restored to the level of 1 mA (148 of FIG. 4a). The adjustment of the voltage levels of the feedback loop causes thevoltage level V_(ERR) at the output terminal B of the differentialamplifier to return to a voltage level of approximately 378 mV (138 ofFIG. 4c ) after a period of ringing. The voltage level V_(FST1) at thenode F has a period of ringing (155 of FIG. 4d ) with spiking to avoltage level of approximately 0.5V prior to settling in to a voltagelevel of approximately 378 mV (154 of FIG. 4d ).

The voltage V_(GATE) at the gate of the PMOS pass transistor P9similarly has period of ringing before settling to the operating voltagelevel of approximately 1.87V (156 of FIG. 4e . The regulated outputvoltage level V_(LDO) present at the output terminal D spikes positivelyby a voltage of approximately 22 mV to a voltage level of approximately2.42V (158 of FIG. 4f ) for a brief period of time. The regulated outputvoltage level V_(LDO) then resumes control at the regulated voltagelevel of approximately 2.40V (160 of FIG. 4f ).

FIGS. 5a-5f are plots of voltage and current levels at variousconnection points within the low dropout voltage regulator of FIG. 2during a 200 ma current transient of an output load current of anaccessory connected to the low dropout voltage regulator of FIG. 2. Thelow dropout voltage regulator has a regulated output voltage levelV_(LDO) of approximately 2.4V in this example (172 of FIG. 5f ) with aload current I_(LOAD) of approximately 1 ma (164 of FIG. 5a ) developedfrom a input voltage level VDD applied to the input voltage terminal Eof approximately 2.7V (162 of FIG. 5b ). The voltage level V_(ERR) atthe output terminal B of the differential amplifier is at approximately378 mV (128 of FIG. 5c ). The voltage level V_(Fst1) at the commonconnection F is set to the voltage level is approximately 700 mV (168 ofFIG. 5d ). This forces the voltage level V_(GATE) at the gate of thePMOS pass transistor P9 to be 1.87V (170 of FIG. 5e ).

The accessory connected to the low dropout voltage regulator increasesits load current I_(LOAD) from the output terminal D to a value ofapproximately 200 mA (174 of FIG. 5a ) that is four times its maximumoutput drive capability. The low dropout voltage regulator will then gointo current limit. As the low dropout voltage regulator is in currentlimit, the feedback loop modifies the potential of internal nodes of lowdropout voltage regulator such that the voltage level V_(ERR) at theoutput terminal B of the differential amplifier drops near the groundreference voltage level or approximately 119 mV (178 of FIG. 5c ). Thevoltage level V_(FST1) at the node F that is the junction of the drainof the NMOS transistor N3, the gate of the NMOS transistor N4, the drainof the PMOS transistor P7, and the source of the PMOS transistor P11, isclamped to a voltage of approximately 2.5V (180 of FIG. 5d ) and thevoltage V_(GATE) at the gate of the PMOS pass transistor P9 reduces toclose to the ground reference voltage level or approximately 134 mV (182of FIG. 5e ). The regulated output voltage level V_(LDO) (144 of FIG. 5f) present at the output terminal D spikes negatively to a voltage levelof approximately 2.35V for a brief period of time and then resumescontrol at the regulated voltage level of approximately 2.40V. The inputvoltage level VDD applied to the input voltage terminal E decreases by avoltage of approximately 70 mV to approximately 2.61V (176 of FIG. 5b ).

When the accessory no longer requires the load current of 200mA, theload current I_(LOAD) is restored to the level of 1 mA (148 of FIG. 5a). The adjustment of the voltage levels of the feedback loop causes thevoltage level V_(ERR) at the output terminal B of the differentialamplifier to return to a voltage level of approximately 378 mV (190 ofFIG. 5c ) after a period of ringing. The voltage level V_(FST1) at thenode F has a period of ringing with spike to the ground referencevoltage level (191 of FIG. 5d ) prior to settling in to a voltage levelof approximately 700 mV (192 of FIG. 4d ).

The voltage V_(GATE) at the gate of the PMOS pass transistor P9similarly has period of spiking (193 of FIG. 5e ) before settling to theoperating voltage level of approximately 1.87V (194 of FIG. 5e ). Theregulated output voltage level V_(LDO) present at the output terminal Dovershoots positively by a voltage of approximately 270 mV to a voltagelevel of approximately 2.67V (196 of FIG. 50 for a brief period of time.The regulated output voltage level V_(LDO) then resumes control at theregulated voltage level of approximately 2.40V (198 of FIG. 5f ).

FIG. 6 is a schematic diagram of a low dropout voltage regulator of thepresent disclosure. The structure and function of the low dropoutvoltage regulator is as shown in FIG. 2 with the exception of theaddition of the local control loop LCL and the transient fault sensecircuit DOSC. The local control loop LCL and the transient fault sensecircuit DOSC sense that the low dropout voltage regulator is inresponding to an input transient voltage fault or output transientcurrent fault. The transient fault sense circuit DOSC is connected tothe drain of the PMOS transistor P11. The gate and drain of the diodeconnected NMOS transistor N11 is connected to the drain of the PMOStransistor P11 and to a first terminal of the first biasing currentsource I₁. The second terminal of the first biasing current source I₁and the source of the NMOS transistor N11 are connected to the groundreference voltage source. The drains of the PMOS transistor P11 and theNMOS transistor N11, the gate of the NMOS transistor N11, and the firstterminal of the first biasing current source are connected to the gateof the NMOS transistor N12. The source of the NMOS transistor N12 isconnected to the ground reference voltage source. The drain of the NMOStransistor N12 is connected to the check terminal G that is connected tothe local control loop LCL. The gate of the PMOS transistor P13 isconnected to the check terminal G and thus to the drain of the NMOStransistor N12. A first terminal of the second biasing current source I₂is connected to receive the input voltage level VDD. The second terminalof the second biasing current source I₂ is connected to the checkterminal G and the drain of the NMOS transistor N12.

The local control loop LCL includes the PMOS transistors P12 and P13.The source of the PMOS transistor P12 is connected to the common sourcesof the differential pair of PMOS transistors P4 and P5. The gate of thePMOS transistor P12 is connected to receive the feedback voltage levelV_(FB) from the common connection A of the series resistors R₁ and R₂.The drain of the PMOS transistor P12 is connected to the source of thePMOS transistor P13 and the source of the PMOS transistor P13 isconnected to the output terminal B of the differential pair oftransistors P4 and P5.

The PMOS transistor P12 essentially forms a differential pair oftransistors with the PMOS transistor P5 to compare the voltagedifferential between the feedback voltage V_(FB) and the referencevoltage level V_(REF). The PMOS transistor P13 is a gating transistorthat is activated when the PMOS pass transistor P9 is forced into thedeep triode operating region due to the transient voltage present at theinput terminal E causing a negative voltage excursion of the inputvoltage level VDD.

FIGS. 7a-7f are plots of voltage levels at various connection pointswithin the low dropout voltage regulator of FIG. 6 during a voltagetransient of the input voltage level VDD applied to the input voltageterminal E. If a line transient (200 of FIG. 5a ) occurs that causes theinput voltage level VDD applied to the input voltage terminal E to havea voltage excursion that approaches or exceeds the regulated outputvoltage level V_(LDO) (205 of FIG. 5f ) present at the output terminalD, the internal nodes of the error amplifier A_(ERR) will be skewed. Inthe present example, the transient of the voltage level VDD is from 3.5Vto 2.97V and back to 3.5V. The rise and fall times for the input voltagelevel VDD are 4 μs. The voltage level V_(ERR) at the output terminal Bof the differential amplifier will now be clamped (210 of FIG. 5b )close to its operating point. As the voltage level V_(Fst1) at thecommon connection F begins to the rise (215 of FIG. 5c ) as result ofthe NMOS transistor N3 being closer to being turned off, thegate-to-source voltage of the PMOS transistor P11 becomes large enoughto turn on the PMOS transistor P11. The voltage level V_(FST1) at thecommon connection F is clamped to a voltage level of approximately 2.4Vor the gate-to-source voltage level (Vgs) of the PMOS transistor P11greater than the reference voltage V_(REF). With the output voltagelevel V_(ERR) of the differential amplifier being a lower voltage suchthat the NMOS transistor N3 is not conducting to the degree that it wasduring operation, the majority of the current flowing through the PMOStransistor P7 will be flowing through the PMOS transistor P11. Thevoltage level V_(Fst1) causes the NMOS transistor N4 to turn on and thusthe voltage level V_(GATE) (230 of FIG. 3d ) at the output terminal C ofthe error amplifier A_(ERR) approaches the voltage level of the groundreference voltage source and the PMOS pass transistor P9 is turned fullyon to be into the deep triode operating region (235 of FIG. 5f ).

The current through the PMOS transistor P11 is passed through the diodeconnected transistor N11 and the current source I₁ and the voltage atthe gate increases sufficiently to turn on the NMOS transistor N12. Thevoltage V_(CHECK) (220 of FIG. 5d ) at the check terminal G decreasesand turns on the gating PMOS transistor P13. The local control loop LCLbegins to regulate the differential amplifier by balancing the currentin the active load of the NMOS transistors N5 and N6 of the differentialamplifier. The current deficit in the PMOS transistor P5 resulting fromthe change in the input voltage level VDD is compensated for by thecurrent contributed through the PMOS transistors P12 and P13. Thiscompensation current forces the voltage level V_(ERR) at the outputterminal B of the differential amplifier to be clamped (210 of FIG. 5b )close to its operating point.

When the line transient is removed (245 of FIG. 5a ) and the voltagelevel VDD applied to the input voltage terminal E returns from theminimum voltage of the transient (2.97V) to the normal applied voltagelevel (3.5V), the error output voltage level V_(ERR) (245 of FIG. 5b )at the output of the differential amplifier takes much less time toreach its desired operating voltage. During this time the PMOS passtransistor P9 comes out of the deep triode operating region (265 of FIG.3d ) and will source the correct current to the output terminal D andthe load capacitor C_(L) and the load resistor R_(L). The output voltagelevel V_(LDO), will return to the regulated output voltage level V_(LDO)of approximately 3.0V and the large voltage overshoot is reduced oreliminated.

The voltage level V_(Fst1) (250 of FIG. 5c ) at the common connection Ffalls back to operating point and the voltage V_(CHECK) (255 of FIG. 5d) at the check terminal G decreases to disable the PMOS transistors P12and P13 of the local control loop LCL. The NMOS transistor N4 is nowoperated in a saturation mode such that the gate voltage V_(GATE) (260of FIG. 5e ) is returned to its operating voltage.

The first biasing current source I₁ is provided to insure that the NMOStransistor N11 is not conducting under normal operations to furtherdisable the PMOS transistors P12 and P13 of the local control loop LCL.

FIG. 8 is a schematic diagram of a low dropout voltage regulator of asecond implementation embodying the principals of the presentdisclosure. The structure and function of the low dropout voltageregulator is as shown in FIG. 2 with the exception of the addition ofthe cascode voltage generator formed of the PMOS transistor P10 and theNMOS transistor N2 a. Additionally, the sources of the PMOS transistorsP1 and P2 of the biasing current source I_(BIAS) and the PMOS transistorP3 of the biasing current source are now connected to the battery supplyvoltage source V_(BAT) at the node H and the diode connected PMOStransistors P6 and P8 and the PMOS load transistor P7 are connected tothe input pass voltage level VDD at the node E.

The cascode voltage as generated by the PMOS transistor P10 and the NMOStransistor N2 a has a lower amplitude than the gate voltage of the PMOStransistor P7. The gate and drain of the diode connected PMOS transistorP10 is connected to the drain of the NMOS transistor N2 a and to thegate of the PMOS transistor P11 at the terminal C. The source of thePMOS transistor P10 is connected to the power supply voltage source VDDat the terminal E. The source of the NMOS transistor N2 a is connectedto the ground reference voltage source. The gate of the NMOS transistorN2 a is connected to receive the biasing voltage V_(NBIAS).

The drain of the PMOS transistor P11, the gate of the NMOS transistorN3, and the first plate of the compensation capacitor C_(COMP) areconnected to the output terminal B of the differential pair oftransistors P4 and P5 of the differential amplifier DIFF1 as the errorvoltage V_(ERR).

When the low dropout voltage regulator goes is responding to input oroutput transient faults, the output voltage decreases causing thefeedback voltage V_(FB) amplitude to become less than the referencevoltage level V_(REF). This causes output voltage level V_(ERR) of thedifferential amplifier DIFF1 to approach the voltage level of the groundreference voltage such that that the transistor N3 is nearly switchedoff. The voltage level V_(Fst1) at the node F increases to the inputpass voltage level VDD to fully turn on the NMOS transistor N4 and thusforcing the voltage level V_(GATE) at the node C to be close to theground reference voltage level thus forcing the PMOS pass transistor P9deep into triode region of operation.

When voltage level V_(Fst1) at the common connection F is approachingthe voltage level of the input pass voltage level VDD, the PMOStransistor P11 begins to conduct and a portion of the current from thePMOS transistor P7 flows via the PMOS transistor P11 into the drain ofthe load NMOS transistor N6 and balances the current in the active loadfor the differential amplifier DIFF1. The voltage level V_(ERR) at theoutput node B of the differential amplifier DIFF1 is prevented frombeing pulled to ground reference voltage source and maintains it closeto the operating voltage level of the differential amplifier DIFF1 whilePMOS pass transistor P9 is forced into deep triode region of operation.This allows the control loop to respond very quickly to control the gateof PMOS pass transistor P9 and minimizes any overshoot of the outputvoltage level V_(LDO) at the output node D, when the transient isremoved from the input pass power supply voltage source VDD.

FIGS. 9a-9e are plots of voltage levels at various connection pointswithin the low dropout voltage regulator of FIG. 8 during a voltagetransient of the input voltage level VDD applied to the input voltageterminal E. If a line transient (400 of FIG. 7a ) occurs that causes theinput pass voltage level VDD applied to the input voltage terminal E tohave a voltage excursion that approaches or exceeds the regulated outputvoltage level V_(LDO) (405 of FIG. 7f ) present at the output terminalD, the internal nodes of the error amplifier A_(ERR) are skewed. In thepresent example, the transient of the input pass voltage level VDD isfrom approximately 3.5V to approximately 2.97V and back to approximately3.5V. The rise and fall times for the input pass voltage level VDD are 4μs. The voltage level V_(ERR) at the output terminal B of thedifferential amplifier DIFF1 will now be forced (410 of FIG. 7b ) closeto its operating point. The voltage level V_(Fst1) at the commonconnection F begins to the rise (415 of FIG. 7c ) as result of the NMOStransistor N3 being closer to being turned off. The cascode voltagelevel V_(CAS) at the output terminal G of the cascode voltage generatorand connected to the gate of the PMOS transistor P11 becomes largeenough to turn on the PMOS transistor P11. The voltage level V_(FST1) atthe common connection F is clamped to a voltage level of approximately2.4V. The current flowing through the PMOS transistor P7 now flowsthrough the PMOS transistor P11 and into the drain of the load NMOStransistor N6. The compensation current from the PMOS transistor P7forces the error output voltage level V_(ERR) at the output terminal Bof the differential amplifier DIFF1 to be clamped (410 of FIG. 7b )close to its operating point.

The voltage level V_(Fst1) causes the NMOS transistor N4 to turn on andthus the voltage level V_(GATE) at the output terminal C of the erroramplifier A_(ERR) approaches the voltage level (430 of FIG. 7d ) of theground reference voltage source and the PMOS pass transistor P9 isturned fully on to be into the deep triode operating region (435 of FIG.7e ).

When the line transient is removed (440 of FIG. 7a ) and the input passvoltage level VDD applied to the input voltage terminal E returns fromthe minimum voltage of the transient (2.97V) to the normal appliedvoltage level (3.5V), the error output voltage level V_(ERR) (445 ofFIG. 7b ) at the output terminal B of the differential amplifier DIFF1takes much less time to reach its desired operating voltage. During thistime the PMOS pass transistor P9 comes out of the deep triode operatingregion and will source the correct current to the output terminal D andthe load capacitor C_(L) and the load resistor R_(L). The output voltagelevel V_(LDO), will return to the regulated output voltage level ofapproximately 3.0V (465 of FIG. 7e ) and the large voltage overshoot isreduced (470 of FIG. 7e ) to approximately 20 mV.

The voltage level V_(Fst1) (450 of FIG. 7c ) at the common connection Ffalls back to operating point. The NMOS transistor N4 is now operated ina saturation mode such that the gate voltage V_(GATE) (460 of FIG. 7e )is returned to its operating voltage.

FIGS. 10a-10f are plots of voltage and current levels at variousconnection points within the low dropout voltage regulator of FIGS. 6and 8 during a 200 ma current transient fault condition of an outputload current of an accessory connected to the low dropout voltageregulator of FIGS. 6 and 8. The low dropout voltage regulator has aregulated output voltage level V_(LDO) of approximately 2.4V in thisexample (504 of FIG. 10f ) with a load current I_(LOAD) of approximately1 ma (502 of FIG. 10a ) developed from a input voltage level VDD appliedto the input voltage terminal E of approximately 2.7V (500 of FIG. 10b). The voltage level V_(ERR) at the output terminal B of thedifferential amplifier is at approximately 378 mV (506 of FIG. 10c ).The voltage level V_(Fst1) at the common connection F is set to thevoltage level is approximately 700 mV (508 of FIG. 10d ). This forcesthe voltage level V_(GATE) at the gate of the PMOS pass transistor P9 tobe 1.87V (510 of FIG. 10e ).

The accessory of the low dropout voltage regulator increases its loadcurrent I_(LOAD) from the output terminal D to a value of approximately200 mA (514 of FIG. 10a ) that is four times its maximum output drivecapability. The low dropout voltage regulator will then go into currentlimit. As the low dropout voltage regulator is in current limit, thefeedback loop modifies the potential of the new control loops of the lowdropout voltage regulator of FIGS. 6 and 8 such that the voltage levelV_(ERR) at the output terminal B of the differential amplifier does notdrop near the ground reference voltage level. The voltage level V_(ERR)at the output terminal B of the differential amplifier is now maintainedclose to its operating condition or approximately 351 mV (520 of FIG.10c ). The voltage level V_(FST1) at the node F is clamped to a voltageof approximately 2.27V (522 of FIG. 10d ) and the voltage V_(GATE) atthe gate of the PMOS pass transistor P9 reduces to close to the groundreference voltage level or approximately 142 mV (524 of FIG. 10e ). Theregulated output voltage level V_(LDO) present at the output terminal Dspikes negatively to a voltage level of approximately 2.35V (512 of FIG.10f ) for a brief period of time and then resumes control at theregulated voltage level of approximately 2.40V (526 of FIG. 10f ). Theinput voltage level VDD applied to the input voltage terminal Edecreases by a voltage of approximately 70 mV to approximately 2.61V(516 of FIG. 10b ).

When the accessory no longer requires the load current of 200 mA, theload current I_(LOAD) is restored to the level of 1 mA (528 of FIG. 10a). The adjustment of the voltage levels of the new control loops of thelow dropout voltage regulator of FIGS. 6 and 8 causes the voltage levelV_(ERR) at the output terminal B of the differential amplifier to returnto a voltage level of approximately 378 mV (534 of FIG. 10c ) after aperiod of ringing (532 of FIG. 10c ). The voltage level V_(FST1) at thenode F has a period of ringing with spiking to the ground referencevoltage level (536 of FIG. 10d ) prior to settling in to a voltage levelof approximately 700 mV (538 of FIG. 4d ).

The voltage V_(GATE) at the gate of the PMOS pass transistor P9similarly has period of spiking (540 of FIG. 10e ) before settling tothe operating voltage level of approximately 1.87V (542 of FIG. 10e ).The regulated output voltage level V_(LDO) present at the outputterminal D overshoots positively by a voltage of approximately 27 mV toa voltage level of approximately 2.427V (544 of FIG. 10f ) for a briefperiod of time. The regulated output voltage level V_(LDO) then resumescontrol at the regulated voltage level of approximately 2.40V (546 ofFIG. 10f ). Thus, the control loop balances the current in the activeload for the differential amplifier and does not allow the output ofdifferential amplifier to go close to ground but maintains it close toits operating condition to better control the recovery from the currenttransient fault conditions.

FIG. 11 is a flowchart for a method for reducing or eliminatingovershoot at an output terminal of a low dropout voltage regulatorresulting from a transient fault conditions occurring at an inputterminal or an output terminal of the low dropout voltage regulator ofthis disclosure. The method is performed by an overshoot reductioncircuit within the low dropout voltage regulator begins at theactivation (Box 300) of the low dropout voltage regulator. The overshootreduction circuit monitors an input voltage at the input terminal andthe output terminal of the low dropout voltage regulator to sense (Box305) if there is a transient fault condition occurring at the inputterminal or the output terminal. If the transient fault conditionoccurring at an input terminal or an output terminal approaches orexceeds the regulated output voltage level V_(LDO), the internal nodesof the error amplifier of the low dropout voltage regulator are skewed.The overshoot reduction circuit determines (Box 310) if the transientfault condition is present at the input or output terminal of the lowdropout voltage regulator. If the transient fault condition is presentat the input or output terminal, the overshoot reduction circuit has acontrol loop circuit that balances (Box 315) the current within theactive load of the differential amplifier of the error amplifier toclamp the output of the differential amplifier to its normal operatingpoint. If the overshoot reduction circuit with its control loop circuitfor balancing the current in the active load were not present, theoutput of the differential amplifier would be clamped close to theground reference voltage level. This causes the pass transistor of thelow dropout voltage regulator to be into deep triode region of operationand to be delayed in assuming the regulating voltage level.

The transient fault condition ends (Box 320) and the output voltage ofthe error amplifier is set such that the pass transistor respondsquickly to resume the regulation and the overshoot is reduced oreliminated and the overshoot reduction circuit resumes the sensing (Box305) the input voltage level for the presence of the transient. Theovershoot reduction circuit then disables the current balancing.

While this disclosure has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the disclosure. Inparticular, the transistors of the low dropout regulator circuit of FIG.4 are shown as PMOS and NMOS field effect transistors. The conductivitytypes of the PMOS transistors and the NMOS transistors may be exchangedsuch that those shown in FIG. 4 as PMOS transistors may be NMOS and theNMOS transistors may be PMOS transistors with attendant changing of thevoltage levels. Further, the transistors of the low dropout voltageregulator may be P-type and N-type bipolar transistors and be in keepingwith the scope of this disclosure.

What is claimed is:
 1. A control loop circuit for balancing an activeload current in an error amplifier of a low dropout voltage regulatorresulting from removal of a transient fault conditions applied to aninput terminal or output terminal of the low dropout voltage regulator,the control loop circuit comprising: a transient fault sense circuitconfigured to be in communication with a pass transistor of the lowdropout voltage regulator for determining that the low dropout voltageregulator is responding to input or output transient faults andgenerating a transient fault indicator signal indicating that the lowdropout voltage regulator is responding to input or output transientfaults; and a local control loop connected to an output terminal of thetransient fault sense circuit configured to receive the transientindicator signal communicating that the low dropout voltage regulator isresponding to input or output transient faults; wherein the control loopcircuit is configured such that when the transient fault indicatorsignal indicates that the low dropout voltage regulator is responding toinput or output transient faults, the local control loop balances theactive load current of the differential amplifier that clamps an outputof a differential amplifier at a voltage level approaching an operatinglevel, while a pass transistor of the low dropout voltage regulator ismaintained in deep triode region of operation for insuring fast responsein controlling the gate of the pass transistor to minimize overshootwhen the input terminal or output terminal is returned to its normaloperating voltage or current.
 2. The control loop circuit for balancingan active load current in an error amplifier of claim 1 wherein thetransient fault sense circuit comprises: an input terminal connected toan output driver of the low dropout voltage regulator for indicatingthat the low dropout voltage regulator is responding to input or outputtransient faults; a first transistor of a first conductivity type havinga source terminal connected to a sense point within the output driver ofthe low dropout voltage regulator, a gate connected to a referencevoltage source such that when a voltage level at the sense pointincreases sufficiently large, the first transistor of the firstconductivity type is conducting to indicate that the voltage level atthe output of the low dropout voltage regulator is below the requiredregulated voltage and the low dropout voltage regulator is in deeptriode region of operation; a first transistor of a second conductivitytype having a gate and drain connected to the drain of the firsttransistor of the first conductivity type and a source connected to theground reference voltage source; a second transistor of the secondconductivity type has a gate connected to the gate and drain of thefirst transistor of the second conductivity type, a source connected tothe ground reference voltage source, and a drain connected to the outputterminal of the transient fault sense circuit to transfer the transientfault indicator signal to the input terminal of the local control loop;a constant current sink having a first terminal connected to the gateand drain of the first transistor of the second conductivity type, asecond terminal connected to the ground reference voltage source suchthat the constant current sink insures that in normal operation, thefirst transistor of the second conductivity type is not conducting andthe transient fault indicator signal indicates that the transient faultconditions have not occurred; and a constant current source having afirst terminal connected to the input terminal of the low dropoutvoltage regulator and a second terminal connected to the output terminalof the transient fault sense circuit.
 3. The control loop circuit forbalancing an active load current in an error amplifier of claim 2wherein the local control loop comprises: a second transistor of thefirst conductivity type having a gate connected to the input terminal ofthe local control loop to receive the transient fault indicator signal,a drain connected to output terminal of the differential amplifier, anda third transistor of first conductivity type having a drain connectedto a drain of the second transistor of the first conductivity type, asource connected to a biasing current source of the differentialamplifier, and a gate connected to receive a feedback voltage that isscaled from an output voltage level of the output terminal of the lowdropout voltage regulator.
 4. The control loop circuit for balancing anactive load current in an error amplifier of claim 3 wherein when avoltage transient occurs at the input terminal of the low dropoutvoltage regulator, the output of the differential amplifier decreasesand the voltage at the source of the first transistor of the firstconductivity type increases, the first transistor of the firstconductivity type and the second transistor of the second conductivitytype begin to conduct generating the transient fault indicator signalforcing the voltage at the gate of the second transistor of the firstconductivity type to decrease dramatically as the second transistor ofthe second conductivity type begins to saturate causing the secondtransistor of the first conductivity type to conduct, and wherein thevoltage difference between the feedback voltage level and the voltagelevel of the input terminal of the low dropout voltage regulator causesthe third transistor of the first conductivity type to conduct causingthe output voltage level of the error amplifier to be clamped close toits operating level and the low dropout voltage regulator can assumeregulation without a large output voltage overshoot.
 5. A low dropoutvoltage regulator comprising: a control loop circuit for balancing anactive load current in an error amplifier of a low dropout voltageregulator resulting from removal of a transient fault conditions appliedto an input terminal or output terminal of the low dropout voltageregulator, the control loop circuit circuit comprising: a transientfault sense circuit configured to be in communication with a passtransistor of the low dropout voltage regulator for determining that thelow dropout voltage regulator is responding to input or output transientfaults and generating a transient fault indicator signal indicating thatthe low dropout voltage regulator is responding to input or outputtransient faults; and a local control loop connected to an outputterminal of the transient fault sense circuit configured to receive thetransient fault indicator signal communicating that the low dropoutvoltage regulator is responding to input or output transient faults;wherein the control loop circuit is configured such that when thetransient fault indicator signal indicates that the low dropout voltageregulator is responding to input or output transient faults, the localcontrol loop clamps an output of a differential amplifier at a voltagelevel approaching an operating level, while a pass transistor of the lowdropout voltage regulator is maintained in deep triode region ofoperation for insuring fast response in controlling the gate of the passtransistor to minimize overshoot when the input terminal of the lowdropout voltage regulator is ending and the voltage level present at theinput terminal is returned to its normal operating voltage.
 6. The lowdropout voltage regulator of claim 5 wherein the transient fault sensecircuit comprises: an input terminal connected to an output driver ofthe low dropout voltage regulator for indicating that the low dropoutvoltage regulator is responding to input or output transient faults; afirst transistor of a first conductivity type having a source terminalconnected to a sense point within the output driver of the low dropoutvoltage regulator, a gate connected to a reference voltage source suchthat when a voltage level at the sense point increases sufficientlylarge, the first transistor of the first conductivity type is conductingto indicate that the voltage level at the output of the low dropoutvoltage regulator is below the required regulated voltage and the lowdropout voltage regulator is in the deep triode region of operation; afirst transistor of a second conductivity type having a gate and drainconnected to the drain of the first transistor of the first conductivitytype and a source connected to the ground reference voltage source; asecond transistor of the second conductivity type has a gate connectedto the gate and drain of the first transistor of the second conductivitytype, a source connected to the ground reference voltage source, and adrain connected to the output terminal of the transient fault sensecircuit to transfer the transient fault indicator signal to the inputterminal of the local control loop; a constant current sink having afirst terminal connected to the gate and drain of the first transistorof the second conductivity type, a second terminal connected to theground reference voltage source such that the constant current sinkinsures that in normal operation, the first transistor of the secondconductivity type is not conducting and the transient fault indicatorsignal indicates that the transient fault conditions have not occurred;and a constant current source having a first terminal connected to theinput terminal of the low dropout voltage regulator and a secondterminal connected to the output terminal of the transient fault sensecircuit.
 7. The low dropout voltage regulator of claim 6 wherein thelocal control loop comprises: a second transistor of the firstconductivity type having a gate connected to the input terminal of thelocal control loop to receive the transient fault indicator signal, adrain connected to output terminal of the differential amplifier, and athird transistor of first conductivity type having a drain connected toa drain of the second transistor of the first conductivity type, asource connected to a biasing current source of the differentialamplifier, and a gate connected to receive a feedback voltage that isscaled from an output voltage level of the output terminal of the lowdropout voltage regulator.
 8. The low dropout voltage regulator of claim7 wherein when a voltage transient occurs at the input terminal of thelow dropout voltage regulator, the output of the differential amplifierdecreases and the voltage at the source of the first transistor of thefirst conductivity type increases, the first transistor of the firstconductivity type and the second transistor of the second conductivitytype begin to conduct generating the transient fault indicator signalforcing the voltage at the gate of the second transistor of the firstconductivity type to decrease dramatically as the second transistor ofthe second conductivity type begins to saturate causing the secondtransistor of the first conductivity type to conduct, and wherein thevoltage difference between the feedback voltage level and the voltagelevel of the input terminal of the low dropout voltage regulator causesthe third transistor of the first conductivity type to conduct causingthe output voltage level of the error amplifier to be clamped close toits operating level and the low dropout voltage regulator can assumeregulation without a large output voltage overshoot.
 9. A low dropoutvoltage regulator circuit comprising: a local control loop connected toprovide a balancing current to an active load of a differentialamplifier within a low dropout voltage regulator to clamp an outputvoltage level of the differential amplifier near an operational voltagelevel to insure fast response in controlling the gate of a passtransistor to minimize overshoot when a transient fault condition at theinput terminal or the output terminal of the low dropout voltageregulator is ending and the voltage level present at the input terminalor output current level present at the output terminal is returned toits normal operating conditions.
 10. The low dropout voltage regulatorof claim 9 wherein the local control loop comprises: a clampingtransistor of a first conductivity type having a source connected to adriver circuit of the low dropout voltage regulator and a drainconnected to an output of the differential amplifier of the low dropoutvoltage regulator to provide the balancing current to the active load ofthe differential amplifier such that an output voltage at an outputterminal of the differential amplifier is at approximately at theoperating voltage level of the differential amplifier; a diode connectedtransistor of the first conductivity type having a source connected to apass supply voltage source, a gate and drain connected to a gate of theclamping transistor; and a biasing transistor of a second conductivitytype having a drain connected to a drain of the diode connectedtransistor and the gate of the clamping transistor, a gate connected toreceive a biasing voltage level, and a source connected to groundreference voltage source; wherein a voltage developed by the diodeconnected transistor and the biasing transistor at the gate of theclamping transistor is less than the voltage level at the source of theclamping transistor and when the pass supply voltage source has atransient voltage, the clamping transistor is turned on to provide thebalancing current to the active load of the differential amplifier toset output voltage at an output terminal of the differential amplifieris at approximately at the operating voltage level of the differentialamplifier to quickly control a gate of a pass transistor of the lowdropout regulator to minimize an overshoot of an output voltage of thelow dropout voltage regulator.
 11. A method for overshoot reduction forminimizing or eliminating an overshoot at an output terminal of a lowdropout voltage regulator resulting from a transient fault conditionsapplied to an input terminal or output terminal of the low dropoutvoltage regulator, comprising the steps of: activating the low dropoutvoltage regulator; monitoring an input voltage at the input terminal ora current at the output terminal of the low dropout voltage regulator tosense if there is a transient fault condition; providing a Millercapacitance at the output terminal of the differential amplifier of thelow dropout voltage regulator to prevent the output of the differentialamplifier from being discharged to ground; when the transient faultcondition is present at the input terminal or output terminal of the lowdropout voltage regulator, balancing a current within an active load ofa differential amplifier of an error amplifier of the low dropoutvoltage regulator to clamp the output of the differential amplifier toits normal operating point; when the transient fault condition ends,setting the output voltage of the error amplifier such that a passtransistor of the low dropout regulator responds quickly to resume theregulation and the overshoot is reduced or eliminated; sensing of theinput voltage level and the output current level for the presence of thetransient fault conditions; and disabling the current balancing when thetransient fault conditions are not present.